An LCD driving system must include circuits for generation of a common voltage and a group of gamma voltages. Each of the pixels of the LCD panel receives a driving voltage and the common voltage, and a voltage difference therebetween determines the orientation of liquid crystals and therefore the luminance of the pixel. The driving voltages are generated by source drivers. Each source driver receives a pixel value and selects one of the gamma voltages as the driving voltage corresponding to the received pixel value.
FIG. 1 shows a conventional common voltage (denoted as Vcom voltage hereinafter) generation circuit. The conventional Vcom voltage generation circuit is set in a system PCB board. Strings of resistors and a changeable resistor divide the voltage difference between a high reference voltage (denoted as VrefH in FIG. 1) and a low reference voltage (denoted as VrefL in FIG. 1) to generate the Vcom voltage. Then, the Vcom voltage generated is sent out through an output buffer and further to a panel via a tape.
FIG. 2 shows a conventional Gamma voltage generation circuit. The conventional Gamma voltage generation circuit is also set in the system PCB board. Strings of resistors divide the voltage difference between a high reference voltage (denoted as VrefH in FIG. 2) and a low reference voltage (denoted as VrefL in FIG. 2) to generate different Gamma voltages. Then, the Gamma voltages generated are sent out through output buffers and further sent to each source driver chips in a source driver circuit.
Since the Vcom voltage generation circuit and the Gamma voltage generation circuit are set in the system PCB board, the layout of the system PCB board is complicated and is not cost effective.